7886

• UCREFRP Catalog Number: 7886
• Author: Hydrosphere Resource Consultants.
• Title: Yampa River Basin Recommended Alternative, Detailed Feasibility Study
• USFW Year: 1995.
• USFW - Doc Type: Final Report.
• Copyright Material: NO

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Reservoir Site Characterization
mile downstream of the dam. These local changes would probably be judged as significant to a
person with day to day familiarity with the reach but are minor with respect to the Creek as a
whole. Rock outcrops generally limit the lateral extent of the horizontal meander pattern
otherwise.
Cross-section and slope surveying at representative locations, sampling/testing of
streambed alluvium and physical inspection of the area constituted the on-site investigations
which could be economically performed as part of the present study. Visual examination
reveals that Elkhead Creek is a meandering stream traveling through a relatively wide valley of
its own erodible alluvium, constrained occasionally by natural rock outcrops and human
constructed features. With less than a 0.2% hydraulic grade and unstable banks the natural
tendency is for the channel to change in shape and horizontal position relatively easily.
The presence of the dam is partly the cause of these observed changes, but differentiating
the human-caused from natural creek changes is extremely difficult. Streambed/bank samples
taken from 400 and 1,600 feet downstream of the dam illustrate the characteristic impact of a
dam/reservoir on the downstream channel. The more downstream sample of the two, the one
which might be more representative of "natural" conditions, comprised 60% of particles sand-
size or smaller while 28 % of the more upstream sample was sand-size or smaller. The
relatively clear water passing the dam is obviously removing sands, silts and clays from the bed
and banks to restore its sediment load. This accounts for both the degrading and widening
being experienced by the Creek downstream of the dam. This re-suspension of bed material
accelerates the armoring process. Armoring is possible, judging from the streambed samples,
but requires significant degradation. With the information currently available it is difficult to
say whether armoring or development of an equilibrium slope will control the long-term stream
gradient below the dam. While vertical information of pre-dam stream elevations is restricted
to 5-foot contours, the available information and physical observations suggest that
approximately 2 feet of vertical degradation has occurred immediately downstream of the dam
in the 19 years the dam has existed. Vertical degradation does not appear to be limited, so far,
by bedrock.
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